It is desirable from a cost and savings standpoint to replace the use of wrought steel machined parts e.g. axle drive gears for example, with machined parts made from powder metal steel. Such gears fabricated from powder metal steels have performed satisfactorily when tested for mechanical properties and when used in automobiles. Moreover, instead of using 16 pounds of wrought steel to make the 8 pound drive gear blank only 10 pounds of powder metal steel is required. Similar material savings are available for other parts as well.
Unfortunately, the same steel composition used in wrought steel parts cannot be directly substituted for use in powder metal steel parts due to oxidation problems which occur during the manufacture of the powder metal article. Therefore, a suitable steel powder metal composition must be selected which provides comparable mechanical properties but which is not prone to excessive oxidation during the manufacturing steps of the article. The most suitable compositions are ironmolybdenum alloys of a low Mn content of about 0.9% or less (% refers to percent by weight throughout). Parts are made from such alloy powders which have been mixed with carbon in the form of graphite particles and a suitable compacting lubricant. A preferred composition for powder metal ring gears comprises 0.3 to 0.5% MO, about 0.25% C, about 0.2-0.6% Mn, balance substantially iron.
As previously stated, gears fabricated from such powder metal steel perform satisfactorily when tested for mechanical properties and when used in automobiles. However, a problem has been encountered in machining these parts from a high volume, mass production standpoint. For example, when machining standard forged wrought steel gear blanks it is possible to produce about 400 gears before any machine tool changes are required due to wear. On the other hand, when machining powder metal blanks made by current practices, it has been possible to produce only about 50 gears before making machine tool changes. Such poor machinability has made it uneconomical to substitute the use of powder metal steel parts for forged wrought steel parts in mass production situations where machining is necessary to the final part.
In studying the machinability problem in the context of such powder metal compositions it was concluded that the poor machinability of such powder metal steel parts could be attributed to undesirable microstructures produced by the standard heat treatment procedures in preparing the parts. By analyzing the microstructures of such parts it was discovered that standard heat treatment procedures produced undesirable coarse carbides mixed with the desirable pearlite type of carbides in the ferrite matrix of the powder metal steel part. It was thus recognized that a new heat treatment was required for powder metal steel parts which would avoid the formation of the coarse carbides and favor ferrite-pearlite formation. Such a method is provided by this invention.